As taught in Scholl et al U.S. Pat. No. 4,059,714, issued Nov. 22, 1977, entitled "Hot Melt Thermoplastic Adhesive Foam System", in many instances it is useful to apply hot melt adhesives in foam condition. Such foams can be generated by dispersing a gas into liquid hot melt, for example by a gear pump, to form a gas/hot melt mixture in which the gas may be in true solution in the liquid, or it may be dispersed as tiny microbubbles. The gas/liquid mixture is conveyed under pressure from the mixing apparatus through a pressurized conduit, line or hose to a valved dispenser, which may be a gun or a foam dispensing head. Upon dispensing to atmosphere the pressure is released and the gas expands within the liquid to create the foam. A pressurized return or recycle line is usually provided to recirculate the mixture back through the pump, when the dispenser is not dispensing it. In such systems the gas/liquid mixture may be quite hot, and under substantial pressure in the lines. For example, in a hot melt foam generating system the temperature may be on the order of 175.degree. F. to 350.degree. F. for most hot melt adhesives, and the pressure is often on the order of 200 to 500 psi and may be as high as 1500 psi.
It is occasionally necessary to disconnect the delivery and/or return conduits between the pump and the valved foam dispensing head, for example to change or clean the head, replace a worn hose, or for cleaning or servicing and the like. In the past, disconnection of the pressurized conduits in systems containing such gas/liquid mixtures has been a slow, difficult and to some extent even dangerous operation, while the gas/liquid mixture in the conduits is under substantial pressure. The conduits act as accumulators because of the pressurized gas they contain. If that pressure is released abruptly, as when a threaded hose coupling is opened, it expels the mixture rapidly, even violently when the pressure is high, and the mixture blows out of the conduit. (In this respect pressurized gas/liquid systems differ from hydraulic systems, in which the liquid pressure drops very rapidly upon opening a conduit, with little discharge of viscous contents when the pumping means is deenergized.) Because of the heat content, uncontrolled discharge of hot melt adhesive is a safety hazard and unacceptable in an industrial environment. Moreover, because of its high viscosity, the upstream pressure in the conduit drops slowly, and thereby prolongs discharge of the material.
Prior to this invention, it was difficult to rapidly and safely release or "purge" the pressure in pressurized gas/liquid conduits, especially if the liquid contains a large quantity of gas or if the gas has a tendency to remain in admixture with the liquid, as is the case with hot melt adhesives. If a valved outlet is placed in the line and is opened to atmosphere to release the pressure, the material will eject through that valve as a foam around it, which is difficult to handle; moreover, even though the pump is not running, high viscosity of the liquid results in only a slow pressure drop so that material continues to exude for some time. One end of a long hose may still be at substantially higher pressure even though the other end is open. In any event, the "blow out" of material from the hose is wasteful and messy. In practice, this persistence of pressure in a hose, even after the pump has been stopped, has made disconnection of pressure conduits slow, difficult and dangerous. Thus, substantial need has existed for a means of purging the pressure in a controlled, rapid, and safe manner, so that the conduit may be disconnected.